IndexedMemPool: Fix race condition on size_ that can cause the destructor to access...

IndexedMemPool: Fix race condition on size_ that can cause the destructor to access nonexistent slots

Summary:
Contention on allocation when there is exactly one slot remaining can cause size_ to exceed actualCapacity_.
Without taking the min of size_ and actualCapacity_ the destructor may try to destroy out-of-bound slots.

Added a test to test/IndexedMemPoolTest.cpp that failed before the fix and passes after the fix.

Reviewed By: nbronson

Differential Revision: D4837251

fbshipit-source-id: a887487727f17eaf2ba66345f40fc91d2fe3bc00